Hello @Chris Hocking
Here is an example of fault domain usage:
- Each virtual machine in availability set is assigned an update domain and a fault domain by the underlying Azure platform. For a given availability set, five non-user-configurable update domains are assigned by default (Resource Manager deployments can then be increased to provide up to 20 update domains) to indicate groups of virtual machines and underlying physical hardware that can be rebooted at the same time. When more than five virtual machines are configured within a single availability set, the sixth virtual machine is placed into the same update domain as the first virtual machine, the seventh in the same update domain as the second virtual machine, and so on. The order of update domains being rebooted may not proceed sequentially during planned maintenance, but only one update domain is rebooted at a time. A rebooted update domain is given 30 minutes to recover before maintenance is initiated on a different update domain.
There are some best practices:
- Put each application tier into a separate Availability Set. In an N-tier application, don't put VMs from different tiers into the same availability set. VMs in an availability set are placed across fault domains (FDs) and update domains (UD). However, to get the redundancy benefit of FDs and UDs, every VM in the availability set must be able to handle the same client requests.
- The availability set should have the number of fault domains set to 3 and upgrade domains should be set to 20.
Azure supports a maximum of 3 fault domains and 20 upgrade domains. We recommend the maximum of 20 upgrade domains as that will minimize the number of nodes down at any one time.